Machine tools are conventionally provided with a table or bed to which the workpiece is mounted so that a machine operation can be performed on this. The workpiece is normally held down by some form of bolt arrangement, and a common approach is to provide the table or bed with T-slots, as may be seen in FIG. 1. This shows a conventional machine tool 10 having a table 12. The table is mounted on rails 14 for adjustment inwardly and outwardly from the tool. The workpiece 16 is mounted to the table by studs or bolts 18, and the lower ends of these are received in parallel T-slots 20; these extend across table 12 so as to permit adjustment of the position of the workpiece in a direction perpendicular to the rails 14.
As can be seen in FIG. 2, each of the T-slots 20 is made up of a relatively narrow entrance portion 22 at the upper surface of table 12, this being bordered by sidewalls 24, 26, and a wider main portion 30 beneath this, this latter being made up of parallel sidewalls 32, 34 and a bottom wall 36. The lower ends of bolts 18 extend into the T-slots, and it has previously been standard practice to thread these into T-nuts which ride in the slots. These T-nuts have an exterior configuration which conforms generally to that of slots 20, in that there is a relatively narrower upper portion which rides within the entrance slot 22, and a larger diameter lower portion which fits within the main slot portion 30 so as to prevent the nut from being pulled out of the slot. Thus, when the bolt is slackened, the workpiece can be slid back and forth as the T-nut moves longitudinally within the T-slot; then, when the desired position has been reached, the bolt is tightened, drawing the T-nut up against the shoulders 28 of the slots and forcing the workpiece 16 into frictional engagement with table 12.
This conventional arrangement has been in use for many decades, but, despite this, it continues to exhibit several wearisome disadvantages. Perhaps the most serious of these is that the use of T-nuts often leads to a "false clamping" situation: as the upper nuts are repeatedly tightened and loosened, the studs tend to back out of the T-nut until only one or two threads are engaged; this gives the operator the impression that the workpiece is properly secured when the upper nuts are tightened, but the threads may subsequently strip out under machining pressure so that the workpiece suddenly comes loose as the machining operation is in progress. Also, T-nut threads are frequently damaged or become worn, as from cross-threading or material falling into the threads, and this necessitates frequent replacement of the nuts. Furthermore, the bolt (or stud) is usually tightened with a wrench, and as this is done, it will frequently extend through the bottom of the T-nut and damage the bottom wall of the T-slot; this damage tends to accumulate over a period of time, to the extent that the table may eventually become ruined.
Attempts have been made to overcome this latter problem by providing a T-nut with a closed lower end, but the end of the bolt tends to jam into this as it is tightened, ruining the nut and bolt; this problem is so frequently encountered that factories having large machine shop operations have been known to go through thousands of T-nuts every year. Another approach which has been tried is to provide a T-bolt unit, the head of which is received in the T-slot so that a stud extends upwardly from this. Although this obviates the possibility of damaging the slot, many different lengths of T-bolts must be purchased to outfit a single machine tool, and this is both expensive and cumbersome in practice. Also, the threads of the T-bolts are frequently damaged or become worn in use, and then the entire T-bolt must be replaced, which can be quite expensive over time.
There has also been an attempt to avoid these problems entirely by using standard bolts with an adaptor attachment which permits these to engage the T-slots, as is disclosed in U.S. Pat. No. 4,183,387. FIGS. 2-3 illustrate this prior art device, and these show the standard bolt 38 and the adaptor fitting 40 which is mounted to this. As can be seen, the bolt 38, in conventional fashion, is made up generally of an elongate threaded shank portion 42 and a square head portion 44 having flats 46. The adaptor fitting 40, in turn, is made up generally of a flat main plate portion 48 having a central bore 50 through which the shank of the bolt passes, and ear portions which are bent upwardly and downwardly from this; the ear portions 52, 54 along the longitudinal edges of plate portion 48 are downwardly extending, while the ear portions 56, 58 at the ends of the plate are upwardly extending. As is shown in FIG. 2, the upwardly extending ear portions are configured to ride within the entrance slot portion of the T-slot, while the downwardly extending ears ride against the sidewalls of the main slot portion to prevent rotation of the device, and position the upper surface of plate 48 so that this abuts the shoulder 28 of the T-slot when nut 60 is tightened on the end of the bolt.
This arrangement is advantageous in that it permits the use of standard square-headed bolts to mount the workpiece to the table. These bolts are very inexpensive to purchase and replace when worn, and it would be very convenient and inexpensive to provide a supply of these of various lengths for use with a small numer of adaptors, rather than having to purchase many individual T-bolts of various lengths. Unfortunately, this prior art adaptor fitting has been found to exhibit inherent deficiencies which have rendered it generally unacceptable to the trade. Firstly, the plate-like adaptor fitting (which is normally manufactured by cold working of a flat metal plate) simply possesses insufficient strength for this to have an adequate working life; with repeated tightening of the bolt, the adaptor fitting tends to become permanently deformed and must be replaced. Another disadvantage inherent in this construction is that for use with large T-slots the plate material must be relatively thick, and it is impractical to bend the corners of this with a sufficiently small radius that adequate load-bearing surfaces will be provided; for example, a typical large T-slot may have an entrance slot 11/2" wide and a main slot 21/2" inches wide, and would require a 1/4" thick plate for use with a 11/4" hold-down bolt, which material simply can't be bent around sufficiently small radii to render this feasible. Also, because of its configuration, the device can only engage two of the flats on the bolt head with the depending ears, which tends to result excessive wear on the ears and less than positive engagement of the bolt. Furthermore, while the fitting successfully prevents the bolt from rotating and being pulled out of the T-slot when the nut is tightened, it otherwise provides virtually no support to prevent the assembly from tipping or rocking back and forth in the slot; thus, when the nut is slackened, the bolt head and adaptor fitting simply fall downwardly into the slot, and these easily become cockeyed and jammed in the slot when the operator attempts to adjust the position of the workpiece. The consequent inability of this assembly to slide back and forth smoothly represents a source of frustration and wasted time for the operator.
Accordingly, there exists a need for an adaptor which will permit a conventional bolt to be mounted within a T-slot so that this can be used as a hold-down for a workpiece on a machine tool table, yet which is configured so as to have adequate resistance to deformation that this will have an acceptable service life. Furthermore, there is a need for such an adaptor which is configured so that this will support the head of the bolt such that this can be slid back and forth within the T-slot without fear that the assembly will become cockeyed and stuck in the slot.